A wide variety of implantable medical devices for delivering a therapy or monitoring a physiologic condition have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices may employ one or more elongated electrical leads and/or sensors. Such implantable medical devices may deliver therapy and/or monitor the heart, muscle, nerve, brain, stomach or other organs. In some cases, implantable medical devices deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of the lead. A proximal portion of the lead that may be coupled to an implantable medical device housing, which may contain electronic circuitry such as stimulation generation and/or sensing circuitry.
For example, implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic stimulation to the heart by delivering electrical therapy signals such as pulses or shocks for pacing, cardioversion or defibrillation via electrodes of one or more implantable leads. In some cases, such an implantable medical device may sense intrinsic depolarizations of the heart, and control the delivery of such therapy signals to the heart based on the sensing. When an abnormal rhythm is detected, such as bradycardia, tachycardia or fibrillation, an appropriate electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing, cardioversion or defibrillation signals to the heart of the patient upon detecting ventricular tachycardia, and deliver cardioversion or defibrillation electrical signals to a patient's heart upon detecting ventricular fibrillation. Pacing signals typically have a lower energy than cardioversion or defibrillation signals.
Leads associated with such implantable medical devices typically include a lead body extending between a proximal lead end and a distal lead end. The lead body incorporates one or more exposed electrode or sensor elements located at or near the distal lead end. One or more elongated electrical conductors extend through the lead body from a connector assembly provided at a proximal lead end for connection with circuitry within an associated implantable medical device housing and an electrode located at the distal lead end or along a section of the lead body. Each electrical conductor is typically electrically isolated from other electrical conductors and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Cardiac lead bodies tend to be continuously flexed by the beating of the heart. Other stresses may be applied to the lead body during implantation or lead repositioning. Patient movement can cause the route traversed by the lead body to be constricted or otherwise altered, causing stresses on the lead body. The electrical connection between implantable medical device connector elements and the lead connector elements can be intermittently or continuously altered. In some cases, changes in leads or connections may result in intermittent or continuous changes in lead impedance.
Short circuits, open circuits or significant changes in impedance may be referred to, in general, as lead-related conditions. In the case of cardiac leads, sensing of an intrinsic heart rhythm through a lead can be altered by lead-related conditions. Structural modifications to leads, conductors or electrodes may alter sensing integrity. Furthermore, impedance changes in the stimulation path due to lead-related conditions may affect sensing and stimulation integrity for pacing, cardioversion, or defibrillation. In addition to lead-related conditions, conditions associated with sensor devices or sensing circuitry may affect sensing integrity.